The invention concerns what is called a geared turbocompressor, a multistage turbocompressor operated by way of a transmission.
A known turbocompressor of this genus (cf. Dubbel, Taschenbuch fur den Maschinenbau, 14th ed., 1981, 920-21) has four compressor stages with pairs of impellers mounted on two pinion shafts and driven by way of a cogwheel at the center of a single-stage transmission.
The attainable efficiency of a turbocompressor stage depends on intake, revolutions per minute, and head. At ratios of P.sub.A :P.sub.E &gt;60, where P.sub.A is output pressure and P.sub.E is terminal pressure, seven stages are needed, whereas at least eight are necessary for P.sub.A :P.sub.E &gt;80. The flow varies considerably from stage to stage. The speed must accordingly be increased after every two stages to obtain optimal conditions. The pinion shaft in the final stage of an optimal compressor must rotate so much more rapidly than the driveshaft that the requisite transmission ratio of more than 1:24 cannot be obtained in a conventional transmission.
Five-stage and six-stage geared turbocompressors have so far been designed for maximal pressure ratios of P.sub.A :P.sub.E =40. A single-stage transmission must be equipped with three pinion shafts for this purpose. Each pinion shaft drives two directly mounted impellers that constitute two stages. A six-stage compressor with a mean pressure ratio of 1.8 per stage can attain an overall ratio of P.sub.A :P.sub.E =34. Heavily loaded stages with a mean pressure ratio of 2.0 per stage would provide an overall ratio of P.sub.A :P.sub.E =64. Such a compressor would be difficult to control, and the overall efficiency would be very unsatisfactory due to its high Mach numbers.
Pressure ratios higher than 60 can be attained only by adding a seventh or eighth stage if such essential properties as regulation capability and high overall efficiency are to be retained. Attempts to attain maximal efficiency, however, have failed because the maximal ratio attainable in known turbocompressor transmissions is approximately 1:20. For high outputs and electric motors with four poles, this means a maximal speed of 30 000 R.P.M. With this maximum as a point of departure, uniform optimal efficiencies will be impossible in any turbocompressor with flow rates below 60 000 m.sup.3 /h and pressure ratios higher than 60 because the heads in the early stages must be considerably higher than those in the later stages. The early stages will accordingly have the highest energy density and only moderate efficiencies because of the unavoidably high Mach numbers. The seventh stage of a seven-stage turbocompressor with a flow of 55 000 m.sup.3 /h and a pressure ratio of P.sub.A :P.sub.E =64 would have to turn at 35 000 R.P.M. although conventional turbocompressor transmissions can attain only 28 000 R.P.M. The overall efficiency is accordingly not optimal. It is of course possible to provide the fourth pinion shaft with an eighth stage, although the basic problematics of limited speeds would still remain, with the aforesaid consequences.